Preparation of systems of ground particulate material

ABSTRACT

There is disclosed a process for preparing a concentrated aqueous suspension of a finely ground particular material, which process comprises (a) preparing an aqueous suspension comprising at least 20% by weight of the particulate material comprising calcium carbonate in coarse particular form; (b) grinding the suspension formed in step (a) in the presence of a polycarboxylate dispersing agent for the particular material in a grinding process where the pH is not less than 10 to produce an aqueous suspension wherein at least 90% by weight of the particles or the particulate material have an e.s.d. less than 2 μm and 60% by weight have an e.s.d. less than 1 μm; (c) adding an additive comprising an anionic polycarboxylate dispersing agent to the suspension of ground material formed in step (b). The process is characterized in that the aqueous suspension formed in step (c) has a pH which lies within the inclusive range 8.5 to 9.8 and in that, following addition of the anionic polycarboxylate dispersing agent in step (c), the suspension is not ground further. Using the method described, suspension can be prepared which are rheologically stable with time, and which contain a relatively high proportion by weight of a finely divided pigment.

This application is a 371 of PCT/GB96/02268 filed Sep. 12, 1996.

This invention concerns a method for preparing a rheologically stable,concentrated aqueous suspension of an inorganic particulate mineral suchas a finely divided alkaline earth metal pigment.

It is often advantageous to produce and distribute alkaline earth metalpigment or filler materials, especially to the paper and water basedpaints industries, in the form of a concentrated suspension in water. Auser receiving an alkaline earth metal pigment material in this form isable to avoid the high energy and capital costs which would be involvedin the mixing of a dry material with water to form a suspension on hisown premises, and, if the alkaline earth metal pigment is produced by awet route, the producer is able to avoid the costs of complete drying ofthe material. A concentrated aqueous suspension is also easy to handleand convey, and presents no dust problem.

The rheological properties of an aqueous suspension of an alkaline earthmetal pigment depend upon a number of factors including theconcentration of solid material in the suspension, the particle sizedistribution of the solid material, as indicated, for example, by thepercentage by weight of the particles having an equivalent sphericaldiameter (e.s.d.) smaller than 2 μm, and the type and amount ofdispersing agent which is used. Generally, a suspension becomes moreviscous, or less fluid, as the solids concentration is increased, or asthe percentage by weight of the particles having an e.s.d. smaller than2 μm is increased. Also there is an increasing tendency for thesuspension to become more viscous with time on storage. Assuming that aneffective dispersing agent for the alkaline earth metal pigment isavailable, it is generally found that there is an optimum amount of thedispersing agent which must be added to the suspension to give maximumfluidity and maximum rheological stability, other factors being kept thesame.

EP-A-0216002 describes a process of beneficiating calcite includingsubjecting a concentrated slurry of the calcite particles to multistagegrinding by passing the slurry in series from an upstream grinding stagethrough one or more downstream grinding stages wherein each downstreamgrinding stage is charged with a grinding medium having a particle sizefiner than the grinding medium of the preceding upstream grinding stage.Grinding media comprising particles of alumina, alumina/silica, orzirconia/silica are disclosed. A dispersing agent, eg. an aqueoussolution of a water-soluble polyacrylate, such as sodium polyacrylate,is preferably added to the slurry prior to grinding.

U.S. Pat. No. 4,325,514 discloses a grinding method in which a mineralis wet ground in a first grinding chamber in the absence of a grindingmedium. The ground mineral may be further ground in a second grindingchamber in the presence of a grinding medium. In the first grindingchamber a dispersing agent is required. The use of a dispersing agent inthe second grinding chamber is optional.

EP-A-0595723 discloses a grinding process in which a compact mineral(such as calcium carbonate), a lamellar mineral (such as kaolin) and aplastic pigment are co-ground in the presence of a grinding agent inpre-grinding and final grinding steps, both of which are conducted inthe same grinding vessel. The final grinding may be conducted in thepresence of a colloidal protective agent.

We have found that the procedures described in the prior art fortreating inorganic particulate material such as calcium carbonate bygrinding are not optimum and that grinding may be improved by operationin accordance with the present invention.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawing is a diagrammatic flow chart illustrating theprocess embodying the present invention.

The present invention provides a process for forming a concentratedaqueous suspension of a particulate inorganic material such as analkaline earth metal pigment which, when compared with suspensions whichare currently available in commerce, will have a higher solidsconcentration for a given fluidity, percentage by weight of particleshaving an e.s.d. smaller than 2 μm, and dispersing agent dose, or ahigher percentage by weight of particles having an e.s.d. smaller than 2μm for a given fluidity, solids concentration and dispersing agent dose,or a higher fluidity for a given percentage by weight of particleshaving an e.s.d. smaller than 2 μm, solids concentration and dispersingagent dose, or a smaller dispersing agent dose for a given percentage byweight of particles having an e.s.d. smaller than 2 μm, solidsconcentration and fluidity.

According to a first aspect of the present invention, there is provideda process for preparing a concentrated aqueous suspension of a finelyground particulate material, which process comprises:

(a) preparing an aqueous suspension comprising at least 20% by weight ofthe particulate material in coarse particulate form;

(b) grinding the suspension formed in step (a) in the presence of apolycarboxylate dispersing agent for the particulate material;

(c) adding further polycarboxylate dispersing agent to the suspension ofground material formed in step (b);

characterised in that the aqueous suspension formed in step (c) has a pHwhich lies within the inclusive range 8.5 to 9.8; and in that, followingaddition of the polycarboxylate dispersing agent in step (c), thesuspension is not ground further. If appropriate, the suspension ofground material is dewatered, if necessary, to a concentration of atleast 65% by weight of the dry material.

The basis of the present invention is the grinding of an inorganicparticulate material such as an alkaline earth metal pigment, in thepresence of a polycarboxylate dispersing agent, and after the completionof grinding, the addition of further polycarboxylate dispersing agentwhich may or may not be the same as the dispersing agent used in step(a), the pH of the suspension being controlled in step (c) to lie withinthe inclusive range 8.5 to 9.8, preferably 8.5 to 9.5, most preferably8.8 to 9.2. Ways in which the pH may be controlled to lie in therequired range are described hereinafter.

Grinding step (b) may be a single grinding stage or may comprise aseries of grinding stages (multi-stage grinding) as described inEP-P-A-0216002 and U.S. Pat. No. 4,322,514. A dispersant addition sitemay be associated with the or each grinding stage to introducedispersant into the suspension during the grinding operation. The oreach dispersing agent addition site may comprise a grinding vessel ormill at which grinding is also to be carried out in a grinding stage butwe prefer to add the dispersing agent to the suspension before thesuspension reaches the grinding stage, eg. at a site associated with apump for pumping the suspension to the grinding stage which is locateddownstream of it.

In accordance with the invention, an additional dispersant addition sitemay be situated downstream of the last grinding stage to introducefurther dispersant, and if necessary other additive(s), to the groundsuspension.

The stage at which the suspension of ground particulate material isdewatered (if necessary) to a concentration of at least 65% by weight ofthe dry material as not critical. Thus, the post-grinding addition ofpolycarboxylate dispersing agent may be made in step (c) before or afterdewatering.

The inorganic particulate material may comprise an insoluble salt of analkaline earth metal, eg. of calcium. The material after treatment bythe method according co the present invention may be intended for use asa pigment, in particular a paper coating pigment which may be formulatedinto a paper coating composition in a manner known per se.

The concentration of the particulate inorganic material in thesuspension in step (a) is preferably at least 40% by weight, morepreferably in the range of from 60 to 80% by weight.

The method according to the present invention may be carried out as acontinuous process or as a batch process. Preferably, the process is amulti-stage grinding continuous process in which the aqueous suspensionto be treated is continuously passed between grinding stages which arecarried out in separate, interconnected grinding vessels. Such acontinuous process is known as cascade grinding, progressively finersolid material being ground being allowed to pass in the suspension downthe grinding chain. There may be provided between the grinding stagesand optionally after the last grinding stage means for separating thesolid material by particle size, eg. a sieve or screen, so that only afiner separated fraction is passed on.

Each of the grinding stages may be conducted in one or more grindingvessels. For example the first stage may be conducted in a single vesseland one or more of the subsequent stages may be carried out in aplurality of, eg. two or three, vessels connected in series or inparallel.

The grinding carried out at each grinding stage may comprise either (a)medium attrition grinding using a known grinding medium such as granulesof alumina or silica or a mixed oxide system; or (b) autogenous grindingin which the particles grind each other without the assistance of anadditional grinding medium. We prefer that at least the last stage inthe process comprises medium attrition grinding.

In each of the grinding stages wherein the suspension is mediumattrition ground, it is preferably ground with a particulate grindingmedium which comprises particles not larger than 2 mm and not smallerthan 0.1 mm. More preferably the particles of the grinding medium arenot larger than 1 mm and not smaller than 0.25 mm. After the or eachmedium attrition grinding stage, the particulate material being groundmay be separated from the grinding medium.

The addition of the dispersing agent at each addition site may becarried out as a single dose or, especially where the process is acontinuous process, as a plurality of separate doses or as a continuousaddition.

In the method according to the present invention the grinding stages maybe preceded by one or more other treatment steps which may include atleast one preliminary comminution step. For example, the inorganicparticles entering the first grinding stage may have a particle sizedistribution such that at least about 30% (and preferably no greaterthan 75%) have an esd less than 10 μm and not less than 20% have an esdless than 2 μm. These particles may be obtained by treatment of mineralchips, eg. of chalk or marble, up to 100 mm in diameter, normally ofdiameter in the range 10 mm to 30 mm. The treatment of mineral chips mayinclude as a preliminary comminution step treatment in a mill, eg. ahammer mill, or wet autogenous grinding as for example described in U.S.Pat. No. 4,325,514 or by preliminary wet grinding.

In a preliminary wet grinding step, a dispersing agent may or may not beused, depending upon the solids concentration of the suspension. If thesolids concentration is above about 50% by weight a dispersing agentmust normally be used. The dispersing agent in such a preliminary stepis preferably a polycarboxylate.

For example, the preliminary grinding step may be any one of thefollowing:

high solids concentration (above 50% by weight, and preferably in therange from 65% to 80% by weight) wet autogenous grinding with adispersing agent;

low solids concentration (below 50% by weight) wet grinding without adispersing agent, but with or without a grinding medium of chemicalcomposition different from that of the particulate material; in thiscase a dewatering step would be needed as part of the method accordingto the present invention preferably before the first dispersant additionis made; or

dry grinding with no dispersing agent followed by mixing with watercontaining a dispersing agent or to which a dispersing agent issubsequently added to form a high solids concentration suspension.

We prefer that the method according to the present invention is carriedout as an at least three grinding stage grinding process in which thelast three stages are medium attrition grinding stages each carried outafter a separate dispersing agent addition has been made.

These three stages may be preceded by one of the preliminary steps, eg.Preliminary comminution steps, described above. If a preliminarycomminution step is included then we prefer that it is a wet autogenousgrinding step. The overall process will then comprise (at least) a fourgrinding stage process embodying the present invention.

In such three or four stage processes, the particle size distribution ofthe particulate material to be ground in the aqueous suspension whendelivered as a feed suspension in the first of the last three grindingstages is typically such that at least 20% by weight of the particleshave an esd smaller than 2 μm and at least about 30% (preferably nogreater than about 75%) have an esd smaller than 10 μm, as measured bymeans of a SEDIGRAPH 5000 instrument, manufactured by MicrometricsCorporation. The feed from that grinding stage to the next grindingstage preferably has a particle size distribution such that from 50% to80% by weight of the particles have an esd, smaller than 2 μm and atleast 40% have an esd less than 1 μm. Preferably, the feed to the next(last) grinding stage has a particle size distribution such that notless than 90% by weight have an esd less than 2 μm and at least 60% havean esd of less than 1 μm.

The sole or last grinding stage in the method according to the presentinvention may be followed by one or more further treatment stepsadditional to the post-grinding step of adding dispersant as hereindescribed. For example, the aqueous suspension of the ground particulatematerial may be treated by froth flotation as described in GB 2204574 toseparate a fine fraction product. The fine fraction product may haveparticles wherein at least 90% have an esd less than 2 μm and at least60%, preferably at least 75%, have an esd less than 1 μm.

In the preferred three or four grinding stage process embodying theinvention described above we prefer that the feed suspension to thefirst of the last three grinding stages comprises at least 40% byweight, preferably at least 50% by weight and most preferably at least60% by weight (based on its dry weight) of the particulate inorganicsolid material. In practice the aqueous suspension at that stage isunlikely to comprise more than 80% by weight of the particulatematerial.

The sole or last stage or grinding is preferably continued until theparticle size distribution of the ground particulate material is suchthat at least 60% by weight of the particles have an equivalentspherical diameter smaller than 1 μm.

The particulate material treated by the method according to the presentinvention may comprise one or more or a calcium carbonate, a calciumsulphate, a barium sulphate or a magnesium silicate all of a natural orsynthetic, eg. precipitated, origin. The preferred material is a naturalcalcium carbonate obtained from a mineral ore source such as chalk ormarble. The starting ore may contain minor amounts (eg. up to 10% byweight) of other constituents, eg. feldspar and quartz.

Steps (a) and (b) may be combined so that the aqueous suspension isformed in situ at the single grinding chamber or at the first grindingstage if a multi-stage grinding process.

In step (a), the polycarboxylate dispersing agent is preferablysubstantially completely neutralised polycarboxylate. In suchcircumstances, the pH of the suspension during grinding will remain ator near 10.0.

The polycarboxylate dispersing agent added at the various addition sitesin the method according to the present invention, which may be the sameor different materials (as discussed further hereinafter), preferablycomprises a homopolymer or copolymer which contains one or more monomerunits comprising a vinyl or olefinic group which is substituted with atleast one carboxylic acid group, or a water soluble salt thereof.Examples of suitable monomers are acrylic acid, methacrylic acid,itaconic acid, crotonic acid, fumaric acid, maleic acid, maleicanhydride, isocrotonic acid, aconitic acid, mesaconic acid, sinapinicacid, undecylenic acid, angelic acid and hydroxacrylic acid.

The amount of the dispersing agent added at each addition site in themethod according to the present invention may be that represented byfrom 10 to 100 moles of carboxylate per dry tonne of particulatematerial.

During grinding, it is not essential that the treated suspension remains(or even starts) in a fully dispersed condition; nevertheless, duringgrinding, the suspension should be sufficiently fluid for grinding totake place with a reasonable input of energy into the grinder. In thisrespect, the total amount of energy dissipated in the suspension duringthe grinding stages should not normally be greater than 500 kW hours pertonne of dry pigment. It is normally required that the final suspensionwill be fully dispersed, and will not require further additions of adispersing agent to render it fluid.

The weight average molecular weight of the polycarboxylate dispersingagent(s) should be not greater than 20,000, and preferably in the rangefrom 700 to 10,000, as measured by the method of gel permeationchromatography using a low angle laser light scattering detector.

More preferably, the polycarboxylate dispersing agent used in eachaddition is a homopolymer or copolymer consisting of repeating units ofthe general formula: or a water soluble salt thereof,

where R₁ is --H, --Alk, --COOH or --COOAlk ##STR1## and R₂ is --H,--CH₂, COOH or --Alk where Alk represents an alkyl group having from 1to 3 carbon atoms.

A copolymer may also comprise up to 70% of repeating units of thegeneral formula: ##STR2## where R₃ is --H, --C₆ H₅ or --Alk and R₄ is--Cl, --CN, --COOAlk or --OCOAlk

where Alk represents an alkyl group having from 1 to 3 carbon atoms.

Especially preferred are water soluble salts of homopolymers of acrylicacid, methacrylic acid, maleic acid or fumaric acid, water soluble saltsof copolymers or two or more of these monomers, and water soluble saltsof copolymers of maleic acid or fumaric acid with styrene or vinylacetate.

The dispersing agent may be added in the form of an acid or in the formof a salt having as a neutralising cation any one or more of theneutralising cations known in the dispersant art, such as an alkalimetal cation, ammonium cation or an alkyl ammonium cation wherein thealkyl group has not more than 7 carbon atoms.

As described above, it is preferred that the method according to thepresent invention is carried out in at least three grinding stages eachof which is associated with a dispersing agent addition site for addinga dispersing agent to the aqueous suspension. It is preferred that eachseparate addition of dispersing agent constitutes no more than 50% ofthe total amount of dispersing agent added during grinding. Normally,the number of dispersing agent addition sites is not greater than 10.Preferably, where there are D addition sites, each addition shouldconstitute no greater than (2/D×100) % of the total addition during theprocess. More preferably, an equal amount of dispersing agent is addedat each addition site.

As described hereinbefore, the pH of the suspension after the completionof grinding should be controlled to lie in the inclusive range 8.5 to9.8, desirably a pH in the inclusive range 8.5 to 9.5, especially 8.8 to9.2.

In the method of the present invention the pH may be controlled in therequired range in one of various ways which will be evident to thoseskilled in the treatment of inorganic mineral suspensions. Thus, in afirst example of method to ensure that the pH after completion ofgrinding is controlled to lie in the desired range, a dispersing agentwhich is a water soluble acid may be added to the suspension to betreated followed by separate addition of an alkali such as sodiumhydroxide. The amount of alkali added where added at more than onestage, may be reduced from stage-to-stage whereby the pH of the aqueousphase remains in the required range as the number of available cationson the surface of the particle surfaces to be treated increases as theparticle size becomes smaller.

In a second example, a salt of a polycarboxylate dispersing agent, eg.sodium polyacrylate, is added after completion of grinding and theaqueous medium is diluted with water to reduce its pH and is thenpreferably reconcentrated by a known dewatering step, eg. filtration.Dilution and reconcentration in this way are described in the presentapplicant's PCT/GB95/00563 (WO 95/25146).

In a third example, different dispersing agents may be added atdifferent addition stages. Thus, during grinding, a fully neutralisedpolycarboxylate salt may be added as dispersing agent whereas, after thecompletion of grinding, an acid form of the dispersing agent may beadded. Preferably, the basic polymer or copolymer of the differentdispersing agents is the same in each case.

In a fourth example, a combination of different dispersing agents may beadded after the completion of grinding. The different agents maycomprise a fully neutralised polycarboxylate salt and an acid form ofpolycarboxylate which may be added separately one after the other ineither order. Preferably, the basic polymer or copolymer of thedifferent dispersing agents is the same in each case.

In a fifth example, a partially neutralised polycarboxylate may be addedafter the completion of grinding. Partially neutralised polycarboxylatesfor use as dispersing agents for pigments are known per se, eg. fromJP55-40715. The use of such an agent in grinding mineral materials suchas CaCO₃ in an aqueous medium is also described in U.S. Pat. No.4,840,985. However, the process described in that specification isapplied in a single stage and under the specific conditions that theparticulate mineral material treated is coarse, the partiallyneutralised agent is added to the grinding medium before or duringgrinding to assist grinding and the degree of neutralisation of theagent as added is in the range 40 to 96%. In contrast, if in carryingout the method of the present invention it is desired to use the fifthexample wherein a partially neutralised polycarboxylate dispersing agentis added, the dispersant is added to the finely ground material afterthe completion of grinding. We would also contemplate that the degree ofneutralisation of the agent as added is 30% or less. In this example thedispersing agent added prior to and during grinding may suitablycomprise a fully neutralised polycarboxylate. Preferably, the partiallyand fully neutralised polycarboxylates used in the different additionstages comprise the same basic polymer or copolymer, eg. a sodiumpolyacrylate.

The pH may also be maintained in the range 8.5 to 9.8 in one or more ofthe grinding stages in a multi-stage grinding process, especially thelast grinding stage in accordance with our invention the subject of acopending International application of even date. In all stages wherethe pH is to be adjusted to be within a required range, the pH may bemeasured by a pH monitor and the amount of dispersing agent and/or theamount of neutralising agent (if such an additional agent is employed)may be adjusted until the required pH is reached. This adjustment may becarried out automatically using a mechanised addition device workingunder closed loop control using the measured pH value.

In each of the first to fifth examples of controlling the pH in therequired range in the process according to the present invention, thedispersing agent and other additive(s) (if any), eg. alkali in thesecond example, are desirably added whilst the aqueous suspension isbeing stirred or agitated. This will ensure that the required pH in theaqueous suspension is achieved rapidly, almost instantaneously.

The total quantity of dispersing agents used in the method of theinvention should be sufficient to provide a fully deflocculated finalsuspension. In practice, this normally means that the total amount ofthe dispersing agent is at least 0.1% by weight, based on the weight ofdry particulate inorganic material, and generally does not exceed 1.5%by weight, based on the weight of dry particulate inorganic material.More preferably, the quantity of the dispersing agent used in the methodof the invention is at least 0.05% by weight, based upon the weight ofdry particulate inorganic material. Normally, the amount of dispersingagent will not exceed 1.0% by weight at each addition site, the actualamount depending on the particular dispersing agent used, based upon theweight of dry particulate inorganic material. Another basis on which toquantify the amount of dispersing agent to be used is the number ofmoles of carboxyl per tonne of dry pigment. Thus, it is normally foundthat the amount of the dispersing agent added before and during grindingis preferably that represented by from 10 to 100 moles of carboxylateper tonne of dry pigment, and the amount of the dispersing agent addedin step (c) is preferably from 10 to 100 moles of carboxylate per tonneof dry pigment. By way of example, the monomer molecular weight ofcompletely neutralised sodium polyacrylate (CH₂.CH.COONa) is 94 g, andthe monomer molecular weight of poly(acrylic acid) (CH₂.CH.COOH) is 72g. Each of these polymeric dispersing agents has 1 mole of carboxylateper monomer unit. In the case of the poly(acrylic acid) 10 moles ofcarboxylate per tonne of dry pigment is provided by an amount of 0.079%by weight, based on the weight of dry pigment.

The suspension of the ground inorganic material may be dewatered by oneof the methods familiar to those skilled in the art, for example bythermal evaporation, by filtration or by means of a centrifuge. When adispersing agent is added according to step (c) before dewatering isperformed, the dewatering is preferably effected by a process ofmembrane filtration. The suspension is preferably dewatered to aconcentration of at least 75% by weight of dry pigment.

For a better understanding of the present invention, and to show how itmay be put into effect, reference will now be made to the accompanyingdrawing which is diagrammatic flow chart of a process embodying theinvention.

A suspension of an alkaline earth metal pigment which has been subjectedto preliminary comminution is introduced through a conduit 1 to a firstwet attrition grinding mill 3. A solution of a substantially completelyneutralised polyacrylate dispersing agent is also introduced through aconduit 2. The attrition grinding mill is provided with an impeller 4which is rotated on a vertical shaft 5 by suitable driving means (notshown). Before the commencement of grinding the mill is charged with aparticulate grinding medium 6 consisting of grains of diameters in therange of from 0.5 to 1.0 mm. After the alkaline earth metal pigment hasbeen ground for a time sufficient to reduce the percentage by weight ofparticles having an equivalent spherical diameter smaller than 2 μm to asuitable intermediate value, the suspension of alkaline earth metalpigment is withdrawn through a sieve 7 of nominal aperture 0.25 mm,which retains the grinding medium, an outlet 8 and a conduit 9 to asecond attrition grinding mill 11, which is substantially identical tothe first mill. A further quantity of a solution of the substantiallycompletely neutralised polycarboxylate dispersing agent may be added tothe second attrition grinding mill through a conduit 10. After thealkaline earth metal pigment has been ground to the required finaldegree of fineness, the suspension of the pigment is withdrawn throughan outlet 12 where the grinding material is retained on a sieve and aconduit 13 to a mixing tank 14. A quantity of a polycarboxylatedispersing agent is introduced through a conduit 15. The pH is adjustedto approximately pH 9 in one of the ways described above. Additivematerial if required to achieve this, e.g. NaOH, may also be added viathe conduit 15. After mixing for a time sufficient to distribute thepartially neutralised polycarboxylate dispersing agent uniformlythroughout the suspension of the pigment, a fluid and rheologicallystable suspension of the pigment is withdrawn through an outlet 16.

In an alternative embodiment (not shown), further grinding stages,preferably at least three grinding stages in total, are employed eachunder conditions similar to those in the mill 11.

The invention will now be illustrated further by reference to thefollowing example.

EXAMPLE

A sample of a natural ground marble flour having a particle sizedistribution such that substantially all of the particles were smallerthan 53 μm was mixed with water containing 0.2% by weight, based on theweight of dry marble, of a completely neutralised sodium polyacrylatedispersing agent having a weight average molecular weight of 6500. Thesuspension thus formed contained 65% by weight of dry marble. Thesuspension was subjected to attrition grinding using as the grindingmedium silica sand having substantially spherical particles of diameterin the range from 0.5 mm to 1.0 mm. Grinding was continued until theparticle size distribution of the marble was such that 40% by weightconsisted of particles having an equivalent spherical diameter smallerthan 2 μm.

A further amount of 0.1% by weight, based on the weight of dry marble,of the same dispersing agent was then added and the suspension of groundmarble was subjected to a second stage of attrition grinding using thesame grade of silica sand as before. The grinding was continued untilthe particle size of the ground marble was such that 90% by weightconsisted of particles having an equivalent spherical diameter smallerthan 2 μm.

The suspension of the finely ground marble was then divided into twoportions A and B. To Portion A there was added 0.25% by weight, based onthe weight of dry marble, of the same dispersing agent as had been usedin the earlier stages, and to Portion B there was added a polyacrylicacid having a weight average molecular weight of 6500 (as measured inthe fully neutralised form), with no additional additive and to PortionB there was added a polyacrylate dispersing agent having a weightaverage molecular weight of 6500 with adjustment of the pH of thesuspension to approximately pH 9, e.g. pH 8.8 to 9.2. The amount addedwas the such that the number of moles of carboxyl groups added per unitweight of dry pigment was the same as for Portion A. Each portion wasthen dewatered by membrane filtration to a solids concentration of 76%by weight of dry marble, and the viscosity of each suspension wasmeasured immediately after dewatering by means of a BrookfieldViscometer fitted with Spindle No. 3 at a speed of 100 rpm. Furthermeasurements of the viscosity of the suspension were made after thesuspension had been standing for 1 hour and for 24 hours, respectively.

As a comparison, a further suspension was prepared containing 76% byweight of dry marble which had been ground by the grinding medium to aparticle size distribution such that 90% by weight consisted ofparticles having an equivalent spherical diameter smaller than 2 μm.However, in this case, the total amount of 0.55% by weight, based on theweight of dry marble, of the completely neutralised dispersing agentwhich was used for Portion A was added at the commencement of thegrinding operation.

The results are set forth in the following Table.

                  TABLE 1                                                         ______________________________________                                                Viscosity (mPa · s) after time of                                    0 hour     1 hour   24 hours                                          ______________________________________                                        Portion A 185          330      640                                           Portion B 150          210      350                                           Comparison                                                                              220          600      1360                                          ______________________________________                                    

These results show that a suspension containing 76% by weight of marbleground to a particle size distribution such that 90% by weight consistsof particles having an equivalent spherical diameter smaller than 2 μmhas a lower initial viscosity, and a lesser tendency to gel, or increasein viscosity on standing, when it is treated after grinding with apolycarboxylate dispersing agent with the pH adjusted to pH 8.8 to 9.2than when it is treated with an equivalent dose of a completelyneutralised polycarboxylate dispersing agent with no step to control pH.Also both suspensions treated after grinding with an additional dose ofa dispersing agent showed superior rheological properties to anequivalent ground marble suspension which had been prepared by addingthe same total amount of dispersing agent in a single dose at thecommencement of grinding.

What is claimed is:
 1. A process for preparing a concentrated aqueoussuspension of a finely ground particulate material, which processcomprises:(a) preparing an aqueous suspension comprising at least 20% byweight of the particulate material comprising calcium carbonate incoarse particulate form; (b) grinding the suspension formed in step (a)in the presence of a polycarboxylate dispersing agent for theparticulate material in a grinding process in which the pH of thesuspension in at least part of the grinding process is about 10 orhigher to produce an aqueous suspension of a finely ground product ofthe particulate material wherein at least 90% by weight of the particlesof the particulate material have an equivalent spherical diameter ofless than 2 μm and at least 60% by weight of the particles of theparticulate material have an equivalent spherical diameter of less than1 μm; (c) adding to the aqueous suspension of the finely ground productof the particulate material formed in step (b) an additive comprising ananionic polycarboxylate dispersing agent which adjusts the pH of theaqueous suspension whereby the aqueous suspension formed in step (c) hasa pH which lies within the inclusive range 8.5 to 9.8; and whereinfollowing addition of the said additive comprising the polycarboxylatedispersing agent in step (c), the suspension is not ground further.
 2. Aprocess according to claim 1 wherein the aqueous suspension of theproduct of the particulate material of step (c) is dewatered to aconcentration of at least 65% by weight of the dry material.
 3. Aprocess according to claim 1 wherein said grinding process of step (b)comprises a single grinding stage.
 4. A process according to claim 1wherein said grinding process of step (b) is a multi-stage grindingprocess in which an aqueous suspension of the particulate material ispassed to and treated by grinding in each of a series of grindingstages.
 5. A process according to claim 4 wherein each grinding stage ofthe multi-stage grinding process has an associated dispersing agentaddition site or stage which precedes its associated grinding stage. 6.A process according to claim 4 wherein each grinding stage of themulti-stage grinding process has an associated dispersing agent additionsite or stage which is included within its associated grinding stage. 7.A method according to claim 4 wherein of said multi-stage grindingprocess there are at least three grinding stages optionally preceded bya preliminary comminution step, each of the said grinding stages havingan associated dispersing agent addition site or stage.
 8. A methodaccording to claim 7 wherein there are at least three medium attritiongrinding stages preceded by a preliminary comminution step.
 9. A methodaccording to claim 1 wherein a separation step follows the or eachgrinding stage of step (b) whereby a fine fraction of the particulatematerial present in the aqueous suspension delivered from the grindingstage is separated.
 10. A method according to claim 9 wherein theseparation step following the last grinding stage in the case ofmulti-stage grinding, or following the sole grinding stage in the caseof single stage grinding, comprises froth flotation.
 11. A methodaccording to claim 1 wherein the dispersing agents employed at differentstages in the process are the same or different and each comprises awater soluble polymer or copolymer derived from one or more monomersselected from the group consisting of acrylic acid, methacrylic acid,maleic acid and fumaric acid optionally, in the case of a copolymer,together with styrene or vinyl acetate.
 12. A method according to claim1 wherein the pH of the suspension in step (c) lies within the inclusiverange 8.5 to 9.5.
 13. A method according to claim 12 wherein thedispersing agent used in step (b) comprises a fully neutralised salt ofa polycarboxylate.
 14. A method according to claim 12 wherein thedispersing agent used in step (b) and added in step (c) comprises apolycarboxylate having the same polymer structure.
 15. A methodaccording to claim 12 wherein the dispersing agent added in step (c)comprises a water soluble acid and addition of the agent is followed byaddition of an alkali.
 16. A method according to claim 12 wherein thedispersing agent added in step (c) comprises a salt of a water solubleacid and the suspension is diluted with water and subsequentlyreconcentrated.
 17. A method according to claim 12 wherein thedispersing agent added in step (c) comprises (a) a fully neutralisedsalt and (b) a water soluble acid which are added one after the other ineither order.
 18. A method according to claim 12 wherein the dispersingagent added in step (c) comprises a partially neutralised water solublepolycarboxylate in which not more than 30% of the acid groups areneutralised, and that dispersing agent is added prior to the associatedgrinding stage.
 19. A method as claimed in claim 12 wherein thedispersing agents used in steps (b) and (c) each comprises apolyacrylate.